1. Field of the Invention
The present invention relates to a decoding apparatus for receiving a transmitted image or picture signal and reproducing the original image data therefrom and, more particularly, is directed to a decoding apparatus for receiving transmitted image data which has been compression-encoded into image data of a bit number less than that of the original image data and for reproducing the original image data.
2. Description of the Prior Art
Image data are typically quantized with eight bits. To compress the data for transmission, the datum of each pixel is encoded with a bit number smaller than the original quantization bit number. For example, only the upper two bits of an eight-bit pixel datum are transmitted. On the reception side, for the encoded code of the two bits, a reproduction level is represented by the central value in each level.
The dynamic range of the original eight-bit quantization datum (i.e. 0 to 225) is divided into four parts and a 2-bit encoded code is assigned to each part as follows.
______________________________________ Original Encoded Decoded Signal Code Value ______________________________________ 225- (11) 224 192 191- (10) 160 128 127- (01) 96 64 63- (00) 32 ______________________________________
Since the original signal which had 256 levels, is represented by only four levels, there is a problem in that the quantization distortion is large.
In order to eliminate the above-mentioned disadvantage, the present applicant proposed an adaptive dynamic range encoding system (hereunder referred to as "ADRC system") as a compression system for a digital video signal in the level direction (MR 86-43 reported on Dec. 11, 1986 at the Institute of Electronics and Communication Engineers of Japan.
The ADRC system is an encoding system using the same strong time and spatial correlation as a television signal.
Specifically, if an image is divided, each block generally has only a small dynamic range due to local correlation. In the ADRC system, an image is divided into blocks, a dynamic range of each block is obtained, and an adaptive requantizing of a pixel datum is effected according to the dynamic range. As a result, each pixel datum is compressed into a datum with a smaller bit number than its original bit number.
Methods for dividing an image into blocks, that is, the division in the horizontal direction (one-dimensional ADRC), the division in a rectangular area in the horizontal and vertical directions (two-dimensional ADRC) and the division of a spatial area over plural frames (three-dimensional ADRC) have been proposed for example, as disclosed in Japanese Patent Application Disclosure Nos. 61-144990, 61-144989 and 62-926620, which are open to public inspection.
In the three-dimensional ADRC, movement detection between two frames is effected for every block and the data of a subsequent frame are not transmitted in a still block to perform the so-called "frame dropping". In this way, more efficient encoding can be done. In this case, a one-bit-movement-information code is needed for every block. However, in a still area, data compression of 1/2 can be achieved.
A system has been proposed in which the allocated bit number for every block is changed depending on the magnitude of the dynamic range of each block (hereunder called "variable-length ADRC", and which is disclosed in Japanese Patent Application Disclosure No. 61-147689 which is laid open public inspection. Furthermore, a system has been proposed in which the quantization step size is varied depending on the dynamic range of each block as a constant value less than the bit number of the original pixel datum (hereunder called "fixed-length ADRC", and also disclosed in cited Patent Application Disclosure Nos. 61-144990, 61-144989 and 62-926620.
With the above-mentioned ADRC systems, quantization distortion is reduced as compared with the system where only the upper two bits of the eight bit original quantized digital signal are selected and transmitted and an eight-bit datum is reproduced from said two-bit datum. However, in this ADRC system, quantization distortion is still apt to be present in a block with a large dynamic range, due to the decoding of the transmitted data on the decoder side.